This invention relates to the manufacture of boards from materials which are mixtures of particles having a range of sizes, such particles being bonded together to form boards.
More particularly, the invention is concerned with the manufacture of boards composed of particles in the nature of wood bonded by cement. The wood particles used typically comprise a mixture of very fine particles in the nature of wood flour and coarser particles in the form of filaments, the dimensions of the particles in the mixture varying from less than 0.5 mm to more than 30 mm.
This material, with minor additives, is mixed with cement powder and water in such a manner as to cause the wood particles to become effectively "wetted" by the cement before they are laid down into a layer which is compressed and cured to form the finished board. The method by which the wood particles are wetted with cement is known and does not form part of this invention.
However, it is highly desirable to grade the cement-wetted particles in such a way that the finest particles form the surfaces of the finished board, with the coarser particles at the interior. Whilst this effect could be achieved by grading the wood particles before they are coated with cement, the increased complexity and size of the plant required to separate and handle different grades of wood particles, and the duplication of the cement coating process step for each grade of particle make this approach less attractive commercially and in terms of the properties of the board. For this reason, there have been attempts to devise a process for spreading the cement-wetted wood particles in such a manner that they are graded in the required manner as they are formed into a board.
As is known, the spreading of the bulk material consisting of coarse and fine particles, e.g. coarse and fine chips, for the manufacture of wood particle boards, particularly cement-bound wood particle boards, is one of the most important process steps in the industrial mass production of wood particle boards. The spreading of the bulk material onto a forming belt, which is situated in the spreading direction below the spreading device, serves to make the so-called preforms, i.e. a particle cake or particle mat, whose quality is decisive with regard to the final properties of the boards being manufactured. It is of particular importance in this connection that the preform is spread with maximum accuracy and uniformity, since only in this way is it possible for the required bulk material to be used in an economic manner. Further, the demand for accuracy and uniformity of the spreading operation is very significant from the technological point of view so that it is possible to manufacture boards of a predetermined, required quality in terms of structure and strength. Moreover, the uniform distribution of the bulk material over the cross-section of the board is of decisive importance with regard to the degree and uniformity of the strength properties of the finished cement-bonded wood particle board.
The known spreading methods can be divided into two categories in the fabrication of particle boards.
The first group comprises installations in which the bulk material is spread according to the so-called air separation method. In this system, the particle mat coming from the metering hopper reaches a delivery roller, for example a spiked roller, which scatters the particles, thereby causing a disintegration of the mat. The falling particles then fall through an air stream which distributes them onto the forming belt in such a way that the finest particles are deposited in the outer layers and the coarse particles in the middle layer. This process of air separation spreading works satisfactorily for particle boards, but exhibits a considerable disadvantage in the case of cement-bound wood particle boards. The known method only results in satisfactory spreading if the cement-wetted particles do not exceed a certain overall moisture content. Although such cement-wetted particles can still be satisfactorily spread, there result boards with relatively low strength values.
Attempts have already been undertaken to eliminate this disadvantage in the case of three-layer boards in that the dry covering layers are spread by air separation while the middle layer of the board is spread in a different manner so that the moisture in the middle layer can be kept higher. nevertheless, the results are not on a par with the boards in which all layers can be produced with sufficient moisture. In addition, the separate spreading of particles of differing moisture content for the covering layers and the middle layer is only economical in larger plants, especially as it also requires increased investment.
The second group comprises those installations which work with gravity spreading. In this method, the particles coming from the metering hopper are delivered to the rotating spreading device, for example rotating rollers, spiked rollers or similar, e.g. by means of a metering belt, whereby the spreading devices, such as rollers or similar, break up the bulk material on the metering belt and cast it in the direction of the moving forming belt which is underneath the spreading device. The motion of the individual particles results in a certain separating effect in that the heavier coarse particles assume longer trajectories and the lighter, fine particles assume shorter trajectories. The trajectory, as well as the degree to which the bulk material is broken up, are determined by the peripheral speed of the rollers which are provided with spikes or cams.
In many cases use is made of an entire system of such spreading rollers in order to obtain satisfactory breaking up and separation of the particles. Nevertheless, this method has numerous deficiencies, such as uneven separation, short spreading angles, reduction in weight at the edges of the board, build-up at the walls etc. The most disadvantageous effect is that very often individual coarse chips get into the covering layer consisting of finer chips, with the result that the uniform structure of the find covering layers is destroyed. A further disadvantage of the known gravity spreading methods is that the rotation of the spreading devices results in uncontrollable air streams which entrain in particular the fine and superfine particles so that there is inaccuracy of spreading. The uncontrolled air streams also result in undefinable flow conditions in the spreading space above the forming belt. All these disadvantages have an even greater impact when spreading cement-bound wood particle boards.
One particular process as described in British Patent Specification No. 2136837 involves allowing the cement-wetted wood particles to fall onto a travelling belt whilst subjecting the material to a forced air-flow in order to effect at least partial separation of different particle sizes whereby the finer particles are drawn downwardly by such air-flow and brought to the belt by a shorter route than the coarser particles, which are themselves projected forwardly and deflected downwardly off a baffle so as to follow a longer route than the finer particles and settle on top of the finer particles deposited on the belt by the shorter route.
In our experience, this process is capable of improvement and it is an object of the invention to provide an improved method and apparatus for forming bonded particle boards. More particularly the object of the invention is to create a method of spreading bulk material containing lignocellulose and consisting of coarse and find particles, particularly cement-wetted particles, said method avoiding the aforementioned disadvantages and in which the fine and coarse particles are separated uniformly and with a fine transition from which other over a large area so that the occurrence of internal stresses during pressing is largely reduced and the stability of the boards and their strength characteristics are enhanced. The object of the invention also includes the creation of a device enabling the method according to the invention to be implemented.
Accordingly the first aspect of the invention resides in a method of spreading a particulate material comprising particles having a range of dimensions to form a layer in which the particles are graded according to their dimensions, comprising the step of initially projecting the particulate material in a first generally horizontal direction and allowing the material to fall gravitationally onto the moving carrier whilst subjecting the material a controlled air-flow, characterised by the steps of allowing substantially all of the particles to fall onto the carrier without interacting with any fixed surface and subjecting the falling particles to a through-flow of air immediately above the carrier in a second direction opposite to that in which the particles are initially projected.
We have found, particularly with particles of wood of the kind produced by flaking timber into flakes of 0.35 mm thickness and 30 mm width and then subjecting such flakes to a hammer-milling operation to produce a mixture of wood particles ranging from wood flour, comprising particles of less than 0.5 mm, to filaments of about 30 mm in length, that the finer particles firstly are projected less efficiently in the first direction than the coarser particles so as to effect a first step partial separation and secondly are swept along by the air-flow in the second direction above the belt to a greater extent than the coarser particles to effect an enhanced separation, which is a significant improvement on the prior proposal outlined above.
Preferably the method includes the further step of subjecting the protected particles to a secondary air-flow in the direction of projection at a position adjacent to the point at which the material is projected and before the particles are subjected to said air-flow immediately above the carrier and also the further step of subjecting the falling particles at a position beneath the point at which the material is projected to a circulating air-flow to assist downward movement of the falling particles so subjected.
If the moving carrier is travelling in the first direction, the result is that as the finest particles are carried by the air-flow in the opposite direction they are deposited on the carrier at a position in advance (relative to the direction of carrier travel) of the position at which the material is projected in the first direction, so as to form a layer of very fine particles directly on the carrier. Successively coarser particles are deposited further along the belt to build up a layer with the coarsest particles on top.
This layer may be employed without further addition for the manufacture of particle board having a smooth surface on one face only, but normally, in a second stage, further material will be added by projecting it at a second spreading station in the opposite direction, i.e. contrary to the direction of travel of the carrier, and subjecting it to an air-flow in this case in the same direction as the travel of the carrier. In this way, the coarsest particles are deposited first onto the existing layer on the carrier and the finest particles form an upper, facing layer. In either case, the deposited layer is then compressed and cured as necessary to enable the particles to bond together.
The velocity of the air-flow immediately above the carrier may be so controlled as to produce a layer of deposited material which is of controlled, i.e. uniform, concave or convex, thickness in a direction transverse to the direction of movement of the carrier.
Preferably the particulate material is delivered at a metered rate to the point at which it is projected, said material being carried by a feed belt at the base of a bin in which said material is stored to a discharge roller whereby the material is discharged from the bin to means for weighing and volumetrically metering said material and feeding it to the point at which it is projected, wherein the depth of material fed to said discharge roller is controlled by a vertically arranged vibrating plate spaced above said feed belt so as to hold back the bulk of the material in the bin and allow a layer of controlled depth to be carried by the feed belt towards the discharge roller.
A second aspect of the invention resides in apparatus for spreading a particulate material comprising particles having a range of dimensions to form a layer in which the particles are graded according to their dimensions, comprising means for moving a carrier in a generally horizontal direction, a hood arranged over the path of the carrier and defining a main air inlet and a main air outlet at spaced positions immediately above the path of the carrier, means for projecting said material into said hood in a first generally horizontal direction at a position spaced above the path of the carrier, the hood being so shaped and dimensioned as to allow substantially all of the particles to fall gravitationally onto the carrier without interacting with any fixed surface, and means for causing air to flow through the hood from said main air inlet and immediately above the path of the carrier to said main air outlet so as to establish a main air-flow through the hood in a second direction opposite to that at which the material is initially projected.
The apparatus may further comprise a delivery belt for said particulate material which terminates at a material inlet formed in the hood at a position above and between the main air inlet and main air outlet.
The means for projecting the material may comprise an input roller which affords a plurality of radially extending elements which engage the material on the delivery belt, the input roller rotating in such a direction and at such a speed as to project the material into the interior of the hood in said first direction.
The roller preferably comprises a plurality of axially spaced toothed discs.
The larger, generally filamentous particles tend to be engaged by the individual teeth of such a roller and effectively projected in the required direction by virtue of rotation of the roller, whereas the finer particles, more in the nature of wood-flour, tend to flow between the individual strands, with the result that they acquire less horizontal velocity. Thus, the coarser particles are thrown forward relative to the finer particles.
In accordance with a further feature of the invention, the roller is provided with inserts of a hard material, such as tungsten carbide, at the tip of each tooth. This has been found to give rise to a great increase in the working life of such a roller thereby consistently projecting the material over longer periods of operation.
A forced-air circulation within the hood may be employed to draw the finer particles downwards more positively. For this purpose, a toothed roller may be disposed immediately below the material inlet, the roller being rotated in such a direction as to induce a downward air flow in the region forwardly of the end of the delivery belt relative to the direction in which the particles are projected. This enhances the separation of the fine and coarse particles. Preferably such toothed roller comprises a plurality of axially spaced toothed discs.
Moreover, to reduce an observed tendency for such a roller to impose a component of axial flow on the particles, a further feature of the invention resides in the provision of baffle plates between individual discs at spaced positions along the axis of the input roller. This offsets an observed tendency for the material to drift towards one edge of the moving belt under the influence of such a roller thereby, if unchecked, resulting in the deposited material being of greater thickness at one edge of the finished board than at the other.
Whilst, in the prior proposal mentioned above, there is an air inlet between such toothed roller and the underside of the delivery conveyor, we have found, in accordance with a preferred feature of the invention, that it is preferable to close off the space between the toothed roller and the delivery conveyor, and instead to provide an auxiliary air inlet only at a position above the input roller above the delivery conveyor which projects the particles in said first direction.
In this way, in addition to the main air-flow between the main air inlet and the main air outlet immediately above the moving belt on which the material is deposited, there is a secondary air flow which assists the forward projection of the coarser particles, and a circulating air-flow which assists the downward movement of the finer particles towards the main air-flow.
A further feature of the present invention concerns the metering of the particulate material before it reaches the spreading station.
This feature of the invention relates to a method of metering thin, small-piece bulk material containing lignocellulose, such as wood, bagasse or similar chips, wood fibres etc., particularly for the manufacture of cement-bound wood particle boards, in which the bulk material is discharged from a bin and then volumetrically metered on a circulating belt and brought through a spreading device, such as a spreading roller, onto a forming belt, having a bin for the bulk material which is provided with binding agent, particularly cement, with at least one discharge device, such as discharge rollers, discharge belt or similar, and a downstream metering belt on which is disposed at least one volumetric metering device, for example a skimming roller, which influences the weight and/or the volume of the bulk material, said metering device being followed at the end of the metering belt by a spreading device, such as a spreading roller or similar.
In both the manufacture of hardboards, such as particle boards, particularly cement-bound particle boards, the conventional procedure has been that the particles stored in a bin, such as hopper or silo, and provided with binding agent, or particles mixed with cement in the case of cement-bound particle boards, are brought by a discharge device, for example a discharge roller co-operating with a discharge belt, onto a downstream conveyor belt. The discharged quanity of bulk material is metered by weight either cycle-by-cyle on the entire belt of a belt section or volume metered, and is then spread by a spreading device, for example by air separation or by a gravity spreading device, onto the forming belt which accepts the preform which is subsequently to be pressed. With regard to the quality of the finished boards it is very important that the forms, cakes or mats which are spread onto the forming belt are uniform in terms of height, density and structure (interweaving, felting, interlacing etc) so that the finished board has a structure which is as uniform as possible over its entire cross-section. Therefore, it is important that the particles which are wetted with binding agents, particularly cement, be distributed during manufacture as uniformly as possible on the forming belt in width and length.
The particle board industry employs two known methods, namely the so-called volumetric metering of the bulk material or its metering by weight. It is also known to combine volumetric metering and metering by weight.
For volumetric metering, use is made usually of so-called metering rollers, also called skimming rollers, which are height adjustable in relation to the conveyor belt transporting the bulk material. Depending on the height of such a metering roller in relation to the conveyor belt or metering belt transporting the bulk material below, there results a variable cross-section, a so-called metering slit, through which the bulk material can pass in uniform weight, i.e. volumetrically metered. The speed of the conveyor belt conveying the bulk material and the height of the metering slit for the bulk material, i.e. the height of the metering or skimming roller, result in the volumetric quantity of bulk material which is spread onto the actual forming belt for the formation of the preform, cake or mat.
A considerable disadvantage of this method is that the bulk material accumulates in front of the metering roller which, as already mentioned, is usually in the form of a skimming roller, as a result of which there is a separation, demixing or build-up of the bulk material such that the bottom-most layers of the bulk material on the forming belt exhibit a much higher bulk weight than the upper layers as a result of the fines and superfines which accumulate there and/or as a result of pile-up. This changes the uniform composition and thus the mass-related quantity of bulk material, which considerably affects the uniformity and accuracy of the spread preform and thus the desired uniform composition of the finished board and, consequently, its strength characteristics.
This disadvantage is even more pronounced in the case of cement-bound boards than in the case of particle boards provided with binding agents, such as glue, because in the case of cement-bound boards as a result of the cement component the bulk weights are 11/2 to 21/2 times as great as in the case of particle boards bound with glue. Accordingly, as a result of the greater weight, i.e. the greater mass of the bulk material with cement, the above-described disadvantage is even more pronounced in the case of falling and throwing with the result that there are even greater deviations of the mass per unit of volume.
In the known methods, metering by weight is carried out either continuously, for example by means of a conveyor-type weigher, or intermittently, e.g. by means of a cyclic weigher. In practice, the continuous systems employing a conveyor-type weigher or radioactive weight monitoring are gaining ground. In systems with a conveyor-type weigher the quantity of bulk material per unit of time is precisely determined, but the uniform discharge of the bulk material and its distribution on the forming belt are in no way guaranteed. One of the causes of this is that in the systems with a conveyor-type weigher the bulk material is usually brought onto the conveyor-type weigher by way of volumetric metering with a vibrating conveyor. Since, however, the conveyor-type weigher has a limited length and reacts to the total weight of the bulk material, it cannot cope with the fluctuations in time and/or space of the weight of the bulk material.
The distance in time and space between the pulse generator, i.e. the conveyor-type weigher, and the actuator influenced by this conveyor-type weigher, i.e. the metering roller (skimming roller) results in inevitable non-uniformity and inaccuracy in the flow of bulk material. These irregularities are incorporated directly into the spread preform and thus into the finished board.
The problems are similar withe the known systems which employ radioactive isotopes for measuring the weight of the bulk material. As pulse generators, these measuring devices control either a metering roller (skimming roller), as is known from volumetric metering, or a height-adjustable skimmer, scraper or similar actuator which can be viewed as having the same effect as a metering roller with regard to its operating principle. In both cases, there is a build-up of the bulk material in front of such an actuator with the consequent disadvantages already described in the discussion of volumetric metering.
It has been shown that in both above-described known metering methods the variations of spreading, expressed by the area density, are more than .+-.6% in one and the same board and more than .+-.4% from board-to-board. Such great variations in the raw density in one and the same board and from board-to-board result in considerable disadvantages, firstly in the manufacture of the board, particularly during the actual pressing operation, during the swelling of the board and afterwards during the processing of the boards, for example, during smoothing, cutting, milling etc.
In accordance with a proposal as described in British Specification No. 2136754 bulk material is discharged from a bin and subjected to two successive weighing and volumetric metering steps before application to a spreading station. In this proposal, the initial dispensing of the bulk material from the bin is controlled by a belt conveyor which forms the base of the bin, a discharge roller above the discharge end of the belt conveyor which rotates in such a direction as to assist discharge of the bulk material, and a further roller which rotates in the opposite direction to skim excess material away from the discharge roller and return it to the interior of the bin.
We have found it is preferable to control the depth of material which is fed on the belt to the discharge roller by means of a vertically arranged vibrating plate spaced above the belt at the base of the bin, so as to hold back the bulk of the material in the bin and allow a layer of controlled depth to be carried by the belt towards the discharge roller.
The method and apparatus according to the invention make it possible for particle boards, particularly cement-bound particle board, to be metered with optimum uniformity at the factory with regard to the ready-mixed bulk material such that in its width and in its length, i.e. over the entire cross-section of the board, the bulk material is present in uniform height, density, distribution and structure. In particular, this eliminates the previously accepted demixing and accumulation of the material as a result of volumetric metering and the consequent (particularly in the case of cement-bound wood particle boards) variations of spreading and thus the variations with regard to the strength values of the finished board, particularly the bending and transverse tensile strengths.